1. Field of the Invention
This invention relates, in general, to high-temperature materials and, in particular, to a new class of aromatic phthalonitrile monomers containing multiple imide linkages and their conversion to high-temperature thermosetting polymers and the synthesis thereof.
2. Description of the Prior Art
Polyimides, because of their outstanding thermal stability, have been favored for use in advanced engineering structures. In the past, polyimides were difficult to fabricate because of their insolubility in most of the more common solvents. The solubility problem was partially solved by using a poly(amic)-acid intermediate for product fabrication. During the final fabrication steps, imidization of the poly(amic)-acid is easily accomplished to give the desired end product. While this solved the solubility problem, it did not successfully solve a void problem caused by water liberated during imidization when the poly(amic)-acid was cured. The presence of voids in the final product is very undesirable because they reduce the mechanical properties of the product. The void problem can be solved by the development of fully imidized containing monomers or prepolymers, which have reactive end groups for polymerization purposes (see Keller, U.S. patent application Ser. No. 07/352,327, filed May 16, 1989, incorporated herein by reference). In this way, the water of imidization is removed before cure of the prepolymer. Although this substantially solved the void problem, other undesirable characteristics were still present, such as the existence of thermally unstable aliphatic crosslinking centers.
Phthalonitrile polymers constitute a recent and important class of high-temperature materials, having a wide range of uses, such as composite matrices, adhesives, sealants, and even semiconductors. These polymers are prepared from phthalonitriles in which the linking group between the two ortho dinitrile groups separates the dinitrile groups enough to permit polymerization. Presently several bridging groups are known. Examples include aliphatic and unsaturated groups, aromatic groups, aliphatic and aromatic diamide groups, and aliphatic and aromatic ether, sulfone and ketone groups.